thoughtbot/guides — pull request #277 by creuter (2014-12-19 14:02:04+00:00)
Title: Sass: Prefer mixins to @extends

Extends can cause long, unwieldy selectors if not minded. Compiled file
size is negligible with Gzip. Extending doesn't work across media
queries, and can't gracefully accept arguments.

Further discussion at https://github.com/thoughtbot/bitters/pull/140